Helicopters by their very nature experience vibration, particularly in forward flight when rotor blade flapping is highest. Despite a variety of known methods and systems presently used in helicopters to suppress vibration to acceptable levels, it continues to increase maintenance time and cost, require added weight to assure fatigue strength, reduce riding comfort, and degrade crew performance on extended missions. In some cases, vibration even limits speed, because it increases at higher speeds and can exceed allowable levels while additional engine power is still available. If vibration could be greatly reduced throughout the flight regime, the helicopter would gain immensely in utility.
Most helicopter vibration originates in periodic airloads on the rotor blades. These airloads, which occur both in the thrust direction and in the disk plane, are caused by the asymmetric airflow fundamental to the lifting rotor in forward flight. Other vibration sources, e.g., small differences among blades, periodic wake loads on the fuselage and control surfaces, the engine, and auxiliary equipment, are usually much less significant.
Vibration is currently reduced by several methods. (1) Blade and fuselage resonances with rotor harmonic frequencies, particularly the blade-number multiple harmonics, are avoided in initial design and are detuned as far as possible during development when they are found to contribute significantly to the problem. (2) Fuselage structure is designed, when possible, to place critical locations such as pilot or passenger seats near nodes of the highly excited modes. (3) Passive isolation systems may be used to separate the rotor and transmission from the fuselage, or the payload from the fuselage, at the most troublesome forcing frequency. (4) Passive force absorbers may be installed on the blade roots, the hub, or the fuselage structure to generate vibratory forces which oppose the natural ones at the most troublesome frequency. Some of these absorbers are fine-tuned continuously in flight by automatic (closed-loop) systems.